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Subterranean mammals

About mammals > Adaptations for subterranean life

Across the globe, some 300 (7%) of the extant species of mammals belonging to 54 (5%) genera and representing 10 (7.5%) families of four mammalian orders spend most of their lives in moist and dark, climatically stable, oxygen-poor and carbon dioxide-rich, self-constructed underground burrows, deprived of most sensory cues available above ground.

The subterranean ecotope is safe from predators, but relatively unproductive and foraging is rather inefficient. These mammals are fully specialized for their unique way of life in which all the foraging, mating, and breeding takes place underground. These animals are called “subterranean” (“sub” means under, and “terra” means earth or soil), whereas animals that construct extensive burrow systems for shelter but search for their food (also) above ground are denoted “fossorial” (“fossor” means digger). Of course, there is a continuum from fossorial through facultative subterranean to strictly subterranean lifestyles.

The subterranean niche opened to mammals in the upper Eocene (45–35 million years ago [mya]) and then extended into upper Tertiary (Oligocene and Miocene, i.e., 33.7–5.3 mya) and Quaternary (some two mya) when in the course of global cooling and aridization, steppes, savannas, semideserts, and deserts expanded.

In seasonally dry habitats, numerous plants (the so-called geophytes) produce underground storage organs (bulbs and tubers) that can be a substantial source of food for herbivorous animals. (Underground storage organs of some plants such as potatoes, sweet potatoes, yams, cassava, etc. are among the most important human staple foods.) There have been several waves of adaptive radiation when, independently in space and time, mammals in different phylogenetic lineages occupied the underground niche either to feed on geophytes (in the case of rodents) or to feed on invertebrates, which themselves find food and shelter underground (in the cases of insectivores, armadillos, and marsupial moles). Thus, two morphological and ecological subtypes of subterranean mammals have evolved.

Nevertheless, they all have been subjected to similar environmental stresses and, as a consequence, have much in common. Although the subterranean ecotope is relatively simple, monotonous, stable, and predictable in many aspects, it is very specialized and stressful in others. Consequently, the adaptive evolution of subterranean mammals involves structural and functional changes, which are both regressive (degenerative) and progressive (compensatory) in nature. The mosaic convergent global evolution of subterranean mammals due to similar constraints and stresses is a superb example of evidence for evolution through natural selection, evidence obtained through comparative methods.

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